Security devices in the form of stripes, bands, threads, or ribbons are used extensively in security and value documents, providing visual and/or mechanical means for verifying the authenticity of these documents. These security devices may be either fully embedded or partially embedded in the documents, or mounted on a surface thereof.
Security devices that are at least partially embedded can be applied to a forming fibrous web by introducing the security device into the fibrous web during a wet stage of a paper manufacturing process. However, introduction of security devices into the fibrous web in this stage, while suitable for embedded and partially embedded security devices, have heretofore been impractical for surface applied security devices since the resulting sheet material or document would be susceptible to reduced durability. At this stage, the composition of the forming fibrous web is constituted of pulp or fibers and water and/or other moisture. The fibrous web being substantially wet is such that the amount of pulp or fiber ranges from about 0.2 to about 2.0 percent (%) by weight pulp or fiber, while the amount of water or moisture ranges from about 99.8 to about 98.0% by weight moisture or water. For example, in a wet stage application, security devices can be introduced onto or into a forming fibrous web at the wet end of a Fourdrinier or twin wire paper machine, or against a fibrous web forming cylinder in a cylinder paper machine before that portion of the forming cylinder is immersed in pulp or furnish.
It has been found that during the wet stage introduction of the security device to the forming fibrous web, some of the fibers are displaced as they flow around the security device as it is pressed into the fibrous web. This results in displacement of an amount of fibers, from a sub-region (i.e., a region of the fibrous web located under or beneath the security device) and hinge areas (i.e., regions of the fibrous web located next to edges or sides of the security device) that is enough to affect the interaction of the security device with the fibrous web or with the substrate of the resulting sheet material or document. The resulting concentration of fibers in the sub-region and hinge areas is less than the concentration of fibers in the bulk region. This results in weak connective interaction at the interface of the security device and the substrate of the sheet material or document and in particular results in weak connective interaction at the interfacing surface and/or edges of the security device. During use or circulation of resulting documents, these weak regions produce tears in the sheet materials or documents along the interfacing edges between the security device and the substrate or produce a hinge effect (i.e., separated regions between interfacing edges). Moreover, the document tends to demonstrate backside show-through; that is, the applied security device when applied on one side of the fibrous web will produce a shadow effect that is observable from an opposing side of the fibrous web, any resulting fibrous sheet material or any resulting document. This often requires the use of a backside camouflage coating to address the problem. It has also been observed that said resulting sheet material or document demonstrates a reduction in cross-direction (CD) tensile strength.
One alternative for obtaining a surface applied security device is to apply the security device to a surface of a fully formed fibrous substrate. However, application to a fully formed fibrous substrate is accompanied by other substantial limitations. For example, this limits the thickness range of the security device that can be used. Generally, surface application is limited to the very thinnest of security devices, less than 15 microns. Thicker security devices are generally excluded from such applications at least in part because the resulting caliper differential on a resulting sheet material affects downstream processing. As used herein, the term “caliper differential” refers to the difference in height between an upper surface of the security device and an upper surface of the immediate adjoining bulk-region of the fibrous sheet material. Due to the caliper differential produced with thicker security devices that are introduced either in a dry stage of the paper manufacturing process or in a post application process, downstream processes such as winding, sheeting, stacking, cutting and processing through ATMs are impacted in terms of time and costs. Significantly, stacks produced this way are not press-ready or print ready.
In view of the above, there remains a need for improved sheet materials with surface applied security devices regardless of thickness and for improved processes that can produce these sheet materials.